This invention relates to a method of making a free-standing single layer polyimide film that is approximately equally adhesive on both sides. In particular, it relates to heating a coating on a substrate of a solution of a partially imidized polyamic acid to a temperature high enough to form a film, then hard baking the free-standing film after it has been removed from the substrate.
Adhesive films are used in the electronics industry for a variety of purposes such as, for example, bonding a metal lead frame to an integrated circuit chip. Polyimides are often used to make these films because they are good insulators, are stable at high temperatures, and bond well under heat and pressure. In a current commercial manufacturing process for producing single layer adhesive polyimide films, a solution of a partially imidized polyamic acid is cast onto a low surface tension surface or a release surface and is baked at about 250.degree. C; the resulting polyimide film is peeled from the surface. However, this process results in a film whose top surface bonds better than its bottom surface. That is, after bonding films made by the current process between two identical surfaces, the surface bonded to the top of the film adheres better than the surface bonded to the bottom of the film. This is true regardless of what kind of non-stick substrate is used for casting the solution of partially imidized polyamic acid when the film is prepared. It is desirable for the bottom and top surfaces to be equally adhesive, especially if this result can be achieved by increasing the adhesiveness of the bottom surface.